Page 192 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 167

water; and an underlying, saturated zone that rests upon fresh bedrock. Flows in
these two zones are called `soil water' and `ground water' flow respectively.

Soil water flow
Water rises towards the surface in soil conditions that provide intermolecular
bonding of the water at a sufficient strength to promote capillary attraction. Flow
takes place under tension, gauge pressures are negative and only the capillary
fringe is saturated. Soluble salts and metals in solution are transported and
precipitated within the porous material when the water is vaporised by plant
transpiration or direct evaporation. Granular disintegration of rock surfaces in
arid areas may occur by the swelling of salt crystals when saline solutions soak
into the rock pores. Crystals of salt build up gradually and exert pressures that
eventually break the rock apart. Similar processes can operate along shorelines
under rapidly drying conditions.

Ground water flow
Under ground water flow conditions, motion is induced either by gravitational
or pressure gradient forces. The gravity force is a function of the slope of the
potential energy gradient and the piezometric head. The slope of the grade line
is given by the ratio of the difference in elevation between the level of the
water table and the point of discharge (piezometric head), and the horizontal
distance between the two points. Pressure-gradient forces act where the
thickness of the flow zone decreases in the direction of the flow. One of these
forces may resist motion, depending upon the configuration of the flow zone
(Dingman, 1984). Parameters of pipe line flow, viscous and other energy losses
can usually be calculated closely for constructing an energy grade line
(Chapter 7). For ground water flow, however, energy losses occur abruptly and
are unpredictable; the various relationships are usually determined empirically
as shown in Fig. 3.15.
One important aspect of ground water storage is the relationship between
ground water seepage into springs and stream flow at different levels of
downcutting. The stream bed at level `a' in Fig. 3.16 is located in rocks above
the maximum water table; the stream is ephemeral in that it does not tap any of
the ground water resources and flows only during periods of intense precipita-
tion and run-off. At level `b' the stream lies just above or below the maximum
water table; flow is perennial for much of the time but may fall to a trickle or
cease altogether during extended dry periods. At level `c' the stream lies just
above or below the minimum water table and variations in flow from surface
run-off are smoothed out by the addition of water from the underground storage;
this stream is perennial and flows strongly at all times.